The present invention relates to a direct thermal ink for use in thermal printing applications. More particularly, the present invention relates to a direct thermal ink having a high solids content, an improved process of manufacturing the thermal ink, and process for using the ink.
In the manufacture of direct thermal paper, typically, a substrate such as paper is coated over its entire surface with an aqueous dispersion comprising initially colorless color formers and color developers along with suitable binders which, when heated, combine to form a visible color. When such a coated substrate passes under the print head of a thermal printer, the areas which are activated by the heated print elements of the printer form colored images on the surface of the substrate.
Among the well-known color formers used in direct thermal coating formulations are colorless chromogenic dye precursors such as leuco dyes, triphenyl methanes, diphenyl methanes, xanthene compounds, thiazene compounds, and spiropyran compounds. Well known color developers include phenolic resins such as acetylated phenolic resins, salicylic acid modified phenolic resins, and novolac type phenolic resins.
The color formers and color developers are typically present in the form of finely-divided solid particles which are dispersed in a binder. Thermal coating formulations also typically contain a sensitizer, which is a low melting point solid which, when subjected to heat, melts and becomes a solvent for the color forming and color developing reactants. Other commonly-used ingredients include fillers or pigments, lubricants, whiteners, image-stabilizers and wetting aids.
However, thermal coating formulations currently in use suffer from a number of drawbacks. For example, the sensitizer contained in the formulation is often not uniformly dispersed such that the color developer and color former are in direct contact, which leads to premature color development of the coating when stored prior to use or after application on a substrate. Further, when the sensitizer is not uniformly dispersed, a higher temperature is required to activate unevenly coated areas of the substrate, which creates a grainy or parchment appearance to any image which is formed.
Another disadvantage of thermal coating formulations is that they are coated as a relatively low solids content aqueous solution (i.e., less than about 30% solids by weight) onto a substrate, which requires that the coated substrate be passed through a dryer. Such coating and drying steps require the use of complex and expensive equipment, and also require a large input of energy. For example, conventional thermal papers are typically coated on large paper machines of up to 100 ft. long using Meyer rod, air knife, or blade coating techniques and then passed through large, low temperature air impinged dryers (up to 100 ft. long) to dry the coatings. Such coating formulations cannot be applied by processes which employ smaller printing and drying equipment, such as, for example, a flexographic printing process, because of their high water content, low solids content and low viscosity.
It is possible to include pigments and/or fillers in the ink to increase the solids content of the coatings and decrease drying time and expense. However, such coating formulations are still unsuitable for use in flexographic printing operations because such pigments and fillers tend to fill up anilox rolls and interfere with print quality.
Accordingly, there remains a need in the art for a direct thermal ink formulation which does not develop color prematurely, which has improved shelf life, which has a high solids content, and which may be easily manufactured and applied to substrates utilizing a variety of processes.